A Low-cost Photovoltaic Energy Harvesting Circuit for Portable Devices

Transcription

1 EEE PEDS 20, Singapore, 5 8 December 20 A Lowcost Photovoltaic Energy Harvesting Circuit for Portable Devices an Y.W. Chung and Yung C. Liang Department of Electrical and Computer Engineering, National University of Singapore Kent Ridge, Singapore 9260 Abstract This paper presents an efficient solar energy harvesting circuit for mobile phone battery chargers which can also be easily adapted for other mobile devices. The energy harvesting circuit is capable of making Maximum Power Point Tracking (MPPT) and also has a builtin battery protection function. A polycrystalline solar panel which supplies an average of 400mW (under sun solar insolation) of power was selected for use in the proposed circuit. MPPT in the circuit is achieved by using the constant voltage tracking principle which is a noncomplex yet highly efficient MPPT control method. t is implemented with purely discrete analog components with ultralow power consumption. The proposed design is extremely compact and is feasible for commercial applications. The solar energy harvesting circuit consumes less than 330µW of power and achieves an overall efficiency of approximately 8090%. ndex Terms Analog Maximum Power Point Tracking Circuit, Mobile Phone Charger, Photovoltaic Energy Harvesting Circuit, Ultralow Power MPPT Circuit However, as shown in Figure, most existing applications employ two separate DC/DC converters; the first converter is used to implement maximum power point tracking to improve overall efficiency whereas a second converter performs output voltage regulation [2]. Additionally, they also require auxiliary sensors to extract the P panel s instantaneous voltage or current readings from a secondary Pilot P panel to enable maximum power point tracking [3]. This paper aims to present a greatly simplified method that employs only one DC/DC converter to perform both MPPT tracking and voltage regulation. Furthermore, no additional sensors are used to realize the Constant oltage MPPT Principle. A detailed study has been conducted to validate the proposed method. A prototype of the solar energy harvester circuit has been developed to facilitate experimental testing of the proposed design and the results and findings are presented in Section. T. NTRDUCTN oday s consumers expect to enjoy a wide range of services on their mobile devices, be it a laptop, smart phone or tablet computer. From traditional voice and data services to faster and more interactive multimedia services, the unprecedented array of features found on present day mobile devices has led to an increased reliance on them. This trend has attracted new efforts to increase and prolong the batterylife of these devices between charges as ultimately, a mobile device is only as portable as its power source. n most present applications, the charging energy is drawn from conventional AC adapters with power plugs and this limits the mobility of mobile devices. Consequently, there is an emerging need to develop autonomous energy sources to supplement these batteries that have a limited life. The application of photovoltaics (P) power generation as mobile phone battery chargers presents a viable solution to the above problem. Photovoltaic refers to the technology used to achieve the direct conversion of sunlight into electricity via a solar cell []. Currently, photovoltaic power generation is extensively used to power remotesensing applications such as wireless sensor nodes that gather sensory information on physical or environmental conditions such as temperature, sound or vibration and communicate them back through a network to a central location [2]. Figure. Conventional twostage DC/DC converter MPPT circuit [3].. DESCRPTN F ENERGY HARESTNG CRCUT WTH PHTLTAC CELLS A. Description of Energy Harvesting Circuit The schematic of proposed energy harvesting circuit is shown in figure 2. The system comprises of a solar panel connected to 4 main modules: (i) A DC/DC Boost Converter (ii) A MPPT Controller Circuit (iii) A Battery Protection Circuit (iv) A Charge Pump Circuit //$ EEE 334

2 M C L M2 D C2 Z UT = BATT Batt in Figure 4 below. t is observed from the results that the output of the solar panel is nonlinear and is greatly influenced by the solar insolation level; the MPP of the solar panel varies as ambient conditions change [5]. Charge Pump UT MPPT Control Circuit Battery Protection Circuit The DC/DC boost converter, which is controlled by the MPPT controller, performs maximum power point tracking as well as voltage regulation. A boost converter is chosen for this task as the solar panel maximum power point (MPP) occurs at approximately.65 while the battery charging is enabled at 5 and thus output voltage boosting is required. n addition, a boost converter is preferred over a buck converter as the former does not require an isolated gate driver circuit which will contribute to additional power losses. The Battery Protection module serves to monitor the mobile phone battery voltage and disconnects the solar panel input from the charging circuit once the battery is fully charged. A Charge Pump is also employed in the circuit to provide the circuit components with the initial startup power supply. t is subsequently disabled when the DC/DC converter output voltage rises above the required value. n the following sections the circuit design, component descriptions and principle of operation of each module mentioned will be discussed in detail. B. Characteristics of a Solar Panel s Figure 2. Block diagram of the proposed energy harvesting circuit. D R sh A solar panel represents the most fundamental power conversion unit of a photovoltaic power generation system [4]. t is a device that directly converts the energy of sunlight into electricity by the photovoltaic effect. As solar panel characteristics are important considerations in the design and development of the energy harvesting circuit, they will be briefly discussed in this section. A solar panel is a nonlinear device and can be modeled as a current source as shown in Figure 3. Preliminary experiments carried out to find the characteristics of a typical polycrystalline solar panel yielded the following results shown R s Figure 3. Solar panel equivalent circuit [4]. R Figure 4. characteristics of a typical solar panel under different solar insolation levels. C. Selection of Solar Panel n this section, the performance of commonly available solar panels are compared and evaluated for their suitability for implementation. TABLE CMPARSN F DFFERENT TYPES F SLAR PANELS AT 000W/M 2 Type of Solar Amorphous Polycrystalline Panel Panel Size Maximum Power utput Maximum Power/Effective Area 53.0mm x 44.7mm 76.0mm x 45.0mm 39 mw mw 58.6 W/m W/m 2 Noting the influence of solar insolation levels on solar panel performance, the maximum output power of an amorphous and a polycrystalline solar panel are measured under the same solar insolation level. As seen from Table, for a solar panel of approximately similar size, the polycrystalline solar panel produces more than 2 times the power produced by the amorphous solar panel and has a maximum power per effective area that is 00% more than the amorphous variant. A similar conclusion on the polycrystalline solar panel s superior performance was also drawn in an extensive study carried out in [6]. The results reflected are largely expected as amorphous solar panels in production today have an efficiency of approximately 57% while polycrystalline solar panels enjoy efficiencies ranging from 35%. Another advantage of polycrystalline solar panels is their longer lifetimes as compared to the amorphous varieties. Therefore the polycrystalline solar panel is favored in this experiment as a result of its significantly higher maximum power per effective area and longer lifetime. 335

3 D. Discussion and selection of MPPT method Performance of any P based system is largely dependent on the system s ability to identify and operate at the solar panel s MPP. As such, many MPPT techniques have been developed and implemented over the years. They range in terms of complexity, cost, hardware and effectiveness [7]. However these various MPPT methods all seek to achieve a common objective, which is to ascertain the solar panel s MPP or MPP and to maintain the circuit operation at that given MPP under varying ambient conditions. This section will aim to briefly discuss 3 popular MPPT methods and describe the MPPT method selection process for this paper. Hill Climbing/Perturbation and bservation (P&) These 2 methods are essentially the same fundamental method but are implemented differently. The Hill Climbing MPPT methods as seen in literatures [8] involve a perturbation in the duty ratio of the power converter while the P& method requires a perturbation in the operating voltage of the photovoltaic cell. The Hill Climbing and P& MPPT methods are most suitable for implementation in applications where performance and reliability are key considerations as optimization is only achieved when implemented with a Digital Signal Processor (DSP) or microcomputer controller which increases the cost of the system. ncremental Conductance The ncremental Conductance method is based on the variation in the slope of a solar cell s power curve at different points [24]. t further relies on the fact that the slope of the solar cell power curve is zero at the maximum power point, positive on the left of the MPP and negative on the right of the MPP. The MPP can then be tracked by comparing the instantaneous conductance (/) to the incremental conductance (Δ/Δ). A major disadvantage of this method is the high cost involved due to the need for microcomputers or DSPs and the need for two sensor probes to constantly measure the instantaneous voltage and current of the solar panel. Fractional C The Fractional C method is a result of the near linear relationship between MPP and C under varying solar insolation and temperature levels [57]. The relationship can be represented by the following equation: MPP k C () The constant of proportionality k is dependent on the solar panel being used and has to be determined empirically. With k known, MPP can be computed using equation (). f the solar panel is used with an application that is exposed to varying ambient conditions and precise tracking of the MPP is desired, C must be measured periodically. The fractional C method is most suited for applications which require a simple and cost effective MPPT method as this method does not require the use of DSP or microcomputer control. Based on the review of the various MPPT methods above, the Hill Climbing/P& method and the ncremental Conductance method are deemed to be unsuitable due to their complexity, high cost and high power consumption that are a result of the need for a microcontroller interface. The MPPT method selected for implementation in this paper is thus a variation of the Fractional C method the Constant oltage Tracking Principle. Figure 5. Solar panel output power curves under different solar intensity levels. Under constant solar intensity, the output voltage of the solar panel can be varied by changing the load resistance terminated at the solar panel. The output power of the solar panel under any solar intensity is thus a function of the solar panel output voltage as shown in Figure 5. From the figure, it can also be observed that the MPP increases with increasing solar intensity; this trend is highlighted by the black diagonal line. Therefore, it is reasonable to conclude that a microcontroller would be required to accurately track the MPP during circuit operation of the solar power mobile phone charger. However, the findings above also highlight that the variation of the MPP under different insolation levels is minimal and any power loss due to MPP mismatch is small. Consider the following: f constant voltage tracking is implemented with the MPP voltage set at.65; At 000 W/m 2 Power loss of less than 3.8mW (or loss of.0% of maximum power) At 800 W/m 2 Maximum power is obtained At 500 W/m 2 Power loss of less than 2.mW (or loss of.2% of maximum power) As it is not efficient and cost effective to employ a digital controller to constantly track the insolation level and adjust the 336

4 MPP voltage accordingly (which the Fractional C method requires), the Constant oltage Tracking Principle which is considered to be one of the simplest control methods, is a suitable alternative as it assumes that any variations in the insolation levels are insignificant to the maximum power drawn from the solar panel. Furthermore, as precision is not critical in the operation of the mobile phone solar energy harvester, the constant reference voltage is an adequate approximation of the solar panel s true MPP [8]. E. MPPT Controller Design The MPPT controller is designed to implement the constant voltage MPPT method discussed above. Under varying solar insolation levels, the MPPT controller circuit will ensure that the solar panel operates close to.65 so that the efficiency of the charging circuit can be maximized. The controller circuit is implemented together with a DC/DC Boost Converter that basically performs as an input voltage regulator to constantly maintain the solar panel s operating voltage at.65. P = The solar panel s operating voltage or P is regulated by controlling the duty cycle (D) of the boost converter. From Faraday s Law, for a DC/DC boost converter: The function D L L DT DT D f D (2) f is different for different types of DC/DC converter but as seen above, for a boost converter, it is given by. Eqn. (2) can then be rewritten as: D M2 f D C v (3) where v C, and the duty cycle D is calculated using f D Eqn. (4) as shown below: D f (4) vc D = BATT Figure 6. DC/DC Boost Converter as an nput Regulator C C2 Therefore a nonlinear feedback linearization block f is included in the control loop so that the resulting u control loop becomes linear and a conventional Proportional ntegral (P) controller can be used to get the desired performance as shown in Fig 7. The above control system can be divided into three sections and is realized by three different perational Amplifiers (p Amps). The P controller is implemented by the pamp ; the proportional gain K is determined by the ratio of R and p R 2 while the integral gain K is given by the capacitor C i 3. The reference signal R is provided by the builtin voltage reference in the MAX 92 which serves primarily to implement the circuit s Battery Protection function. A variable resistor, R v is connected between the solar panel and the input of pamp, it allows for the precise setting of the desired solar panel operating voltage or MPP. The operation performed by pamp is expressed in Eqn. (5). v C Y sr C3 sr C Z 2 3 f X R R Y (5) 2 (6) Subsequently, pamp 2 in the form of a differential amplifier serves as the control loop s feedback linearization block. And its operation is represented by Eqn. (6) shown above. The task of Pulse Width Modulation (PWM) generation is left to pamp 3. The duty cycle of the PWM is proportional to the output of pamp 2, Z, and the frequency is controlled by R 9 and C 4. P R v MPPT Circuit R Figure 7. mplementation Block Diagram R2 C3 R6 DD DD A R3 R A2 P Control DD R4 Feedback Linearization R5 R7 R8 DD C4 A3 G2 R9 PWM Generator Figure 8. MPPT Control Circuit Schematic Diagram 337

5 The timing diagram of the PWM generation is shown in Figure 9. Figure 9. PWM Generation by pamp 2 & 3 The power consumption of the proposed Constant oltage MPPT control method is minimized by the use of ultralow power pamps, MAX 4289 and NCS 2220A. This ensures that majority of the energy harvested by the P panels are channeled to achieve the primary task of the device which is to charge the mobile phone battery. Although the Constant oltage tracking method does not allow for precise tracking of the MPP, the accuracy achieved by the control circuit is sufficient to successfully accomplish the controller s intended objectives. F. Battery Protection Controller Design The Battery Protection controller circuit is realized using the ultralow power comparator MAX 92. This module continuously senses the connected battery voltage and activates MSFET M when the battery has been fully charged. This disconnects the solar panel from the charging circuit and protects the rechargeable battery from the adverse effects of overcharging. These effects include depleted battery capacity and excessively high temperatures which may in turn lead to damages to the battery or electronics in the charging circuit. While the solar panel is disconnected from the charging circuit, the MAX 92 draws its power from the connected rechargeable battery to enable it to keep MSFET M in the N state. G. Charge Pump Circuit A S882Z ultralow voltage operation charge pump is utilized to provide the initial power requirements to the energy harvesting circuit s components. This enables the circuit to be used for charging of devices where power cannot be drawn from the batteries. The charge pump achieves this by storing stepped up electric power drawn from the solar panel into a startup capacitor before discharging it as startup power to the DC/DC converter when the discharge start voltage level is reached. Furthermore, a builtin shutdown function can be activated once the voltage of the connected DC/DC converter rises above the required value, thereby allowing for significant power savings.. EXPERMENTAL RESULTS AND FNDNGS An experimental prototype was developed to assess the performance of the solar energy harvesting circuit under various conditions. The results of the tests are presented in the following section. A. Solar Simulator testing of Energy Harvesting Circuit and MPPT circuit The experimental waveforms obtained are shown in Figures 0 and. The waveforms in both figures are of the P panel operating voltage ( pv ), P panel current ( pv ), Energy Harvesting Circuit output voltage ( o ) and output current ( o ). TABLE EFFCENCY F ENERGY HARESTNG DECE UNDER ARUS SLAR NTENSTES Solar ntensity 000W/m 2 500W/m 2 nput Power P in pv.65 x 240mA= 396mW.65 x 48mA= 244.2mW pv utput Power P The experimental results in Figure 0 are measured under a solar intensity of 000W/m 2. t must be noted from the figure that the solar panel s constant voltage maximum power point of.65 is successfully tracked by the prototype. The energy harvesting device operates close to an efficiency of approximately 82.0% at this insolation level as highlighted by calculations in Table. Figure 0. Waveforms of P panel voltage ( pv ) & current ( pv ), utput voltage ( o ) and current ( o ) under a solar intensity of 000 W/m 2 (at 20µs/div) Similar measurements were taken under a solar insolation level of 500W/m 2 and are shown in Figure below. As predicted, the input power from the solar panel is reduced to 244.2mW under a lower solar intensity. The solar panel o o 3.88 x 85mA= 324.7mW 3.78 x 58.mA= 29.6mW o Efficiency pi n po 82.0% 89.9% 338

6 operating voltage pv maintains at the desired voltage level of.65 and as shown by calculations in Table, the solar panel operates at an efficiency of almost 89.9%. At lower levels of solar insolation, the corresponding battery charging current is also reduced and this results in a lower forward voltage drop across the Schottky diode (D) in the DC/DC boost converter and hence lower power losses. This phenomenon accounts for higher efficiencies achieved at lower solar insolation levels. Figure. Waveforms of P panel voltage ( pv ) & current ( pv ), utput voltage ( o ) and current ( o ) under a solar intensity of 500 W/m 2 (at 20µs/div). CNCLSN n this paper, a lowcost yet highly efficient analog MPPT converter circuit was implemented to realize the Constant oltage MPPT Principle. The developed prototype has demonstrated its ability to constantly operate at the solar panel s MPP to allow maximum power to be harvested regardless of solar insolation levels. By limiting component selection to only analog components with ultralow power consumption, the proposed energy harvester is capable of operating at efficiencies between 8090% and draws less than 330µW of power. An additional Battery Protection feature has also been incorporated into the energy harvesting circuit to enable it to directly charge commercial rechargeable batteries which do not have builtin protection features. ACKNWLEDGMENT The authors would like to thank Ko Ko Win for his technical assistance and discussion in this project. REFERENCES [] Petchjatuporn, P.; Ngamkham, W.; Khaehintung, N.; Sirisuk, P.; Kiranon, W.; A Solarpowered Battery Charger with Neural Network Maximum Power Point Tracking mplemented on a LowCost PCmicrocontroller, nternational Conference on Power Electronics and Drives Systems, PEDS 2005, vol., pp , [2] Ko Ko Win; Xinhui Wu; Dasgupta, S.; Wong Jun Wen; Kumar, R.; Panda, S.K.;, Efficient solar energy harvester for wireless sensor nodes, 200 EEE nternational Conference on Communication Systems (CCS), pp , 79 Nov [3] Dondi, D.; Bertacchini, A.; Larcher, L.; Pavan, P.; Brunelli, D.; Benini, L.;, A solar energy harvesting circuit for low power applications, EEE nternational Conference on Sustainable Energy Technologies, CSET 2008, pp , 2427 Nov [4] Chihchiang Hua; Chihming Shen;, Control of DC/DC converters for solar energy system with maximum power tracking, 23rd nternational Conference on ndustrial Electronics, Control and nstrumentation, 997. ECN 97, vol. 2, pp , 94 Nov [5] Jiyong Li; Honghua Wang;, Maximum Power Point Tracking of Photovoltaic Generation Based on the ptimal Gradient Method, Asia Pacific Power and Energy Engineering Conference, APPEEC 2009, pp. 4, 273 Mar [6] Faiman, D. ; Bukobza, D. ; Kabalo, S. ; Karki,. ; Medwed, B. ; Melnichak,. ; de Held, E. ; ldenkamp, H.;, Amorphous, mono and polycrystalline silicon P modules: a comparative study of their relative efficiencies under various outdoor conditions, Proceedings of 3rd World Conference on Photovoltaic Energy Conversion, 2003, vol. 2, pp , 26 May [7] Esram, T.; Chapman, P.L.;, Comparison of Photovoltaic Array Maximum Power Point Tracking Techniques, EEE Transactions on Energy Conversion, vol. 22, ssue: 2, pp , [8] Weidong Xiao; Dunford, W.G.;, A modified adaptive hill climbing MPPT method for photovoltaic power systems, EEE 35th Annual Power Electronics Specialists Conference, PESC , vol. 3, pp , 2025 Jun [9] eerachary, M.; Senjyu, T.; Uezato, K.;, Maximum power point tracking control of DB converter supplied P system, EE Proceedings Electric Power Applications, vol. 48, ssue 6, pp , Nov [0] Yongho Kim; Hyunmin Jo; Deokjung Kim; A new peak power tracker for costeffective photovoltaic power system, Proceedings of the 3st ntersociety Energy Conversion Engineering Conference, 996. ECEC 96, vol. 3, pp , 6 Aug [] Kasa, N.; ida, T.; wamoto, H.; Maximum power point tracking with capacitor identifier for photovoltaic power system, Eighth nternational Conference on Power Electronics and ariable Speed Drives, 2000, pp , 89 Sep [2] Hussein, K.H.; Muta,.; Hoshino, T.; sakada, M.; Maximum photovoltaic power tracking: an algorithm for rapidly changing atmospheric conditions, EE Proceedings Generation, Transmission and Distribution 995, vol. 42, ssue., pp , Jan [3] TaeYeop Kim; HoGyun Ahn; Seung Kyu Park; YounKyun Lee.; A novel maximum power point tracking control for photovoltaic power system under rapidly changing solar radiation, Proceedings of EEE nternational Symposium on ndustrial Electronics, 200, SE 200, vol. 2, pp. 0 04, 26 Jun [4] Wenkai Wu; Pongratananukul, N.; Weihong Qiu; Rustom, K.; Kasparis, T.; Batarseh,.; DSPbased multiple peak power tracking for expandable power system, Eighteenth Annual EEE Applied Power Electronics Conference and Exposition, APEC '03, vol., pp , 93 Feb [5] Bekker, B.; Beukes, H.J.; Finding an optimal P panel maximum power point tracking method, 7th AFRCN Conference in Africa, AFRCN, 2004, vol. 2, pp , 57 Sep [6] Masoum, M. A.; Dehbonei, H.; Fuchs, E. F.; Theoretical and Experimental Analyses of Photovoltaic Systems with oltage and CurrentBased Maximum Power Point Tracking, Power Engineering Review, EEE, vol. 22, ssue 8, pp. 62, Aug [7] Patterson, D.J.; Electrical system design for a solar powered vehicle, 2st Annual EEE Power Electronics Specialists Conference, 990. PESC '90, pp , 4 Jun 990. [8] Yu, G.J.; Jung, Y.S.; Choi, J.Y.; Choy,.; Song, J.H.; Kim, G.S.; A novel twomode MPPT control algorithm based on comparative study of existing algorithms, Record of the TwentyNinth EEE Conference, Photovoltaic Specialists Conference, Pp , 924 May